Chemical vapour deposition (CVD) is used to deposit thin films or layers on the surface of a substrate or wafer located in a deposition chamber. This process operates by supplying one or more reactive gases to the chamber, often using a carrier gas, to the substrate's surface under conditions that encourage chemical reactions to take place at the surface. For example, TEOS and one of oxygen and ozone may be supplied to the deposition chamber for the formation of a silicon oxide layer on the substrate, and silane and ammonia may be supplied for the formation of a silicon nitride layer. Polycrystalline silicon, or polysilicon, is deposited on the substrate by the decomposition of silane or a shlorosilane by heat.
Gases are also supplied to an etch chamber to perform selective etching of areas of the deposited layers, for example during the formation of electrodes and the source and drain regions of a semiconductor device. Etching gases can include the perfluorinated (PFC) gases such as CF4, C2F6, C3F8, and C4F8, although other suitable etchants include fluorine, NF3, SF6 and hydrofluorocarbon gases, such as CHF3, C2HF5 and CH2F2. Such gases are commonly used to form an opening in a region of a nitride or oxide layer formed over a polysilicon layer and which is exposed by a photoresist layer. Argon is generally also conveyed to the chamber with the etching gas to provide a facilitating gas for the process being conducted in the etch chamber.
During such an etch process, there is typically a residual amount of the gas supplied to the etch chamber contained in the exhaust gas drawn from the etch chamber by a vacuum pump, together with by-products from the etching process, such as SiF4 and COF2, and inert gases such as Ar. Additional nitrogen is often added to the exhaust gas as a purge gas for the vacuum pump.
The perfluorinated gases mentioned above are greenhouse gases, and so before the exhaust gas is vented to the atmosphere, an abatement device such as a thermal processing unit (TPU) or plasma abatement device is provided to convert the PFC gases into water-soluble hydrogen fluoride, and to convert SiF4 into SiO2. The gas stream is subsequently conveyed to a scrubbing unit, wherein the HF is dissolved in water supplied to the scrubbing unit.
Our co-pending US patent application 2006/0101995 A1, the contents of which are incorporated herein by reference, describes apparatus for subsequently treating the acidic HF solution formed in the scrubbing unit. The HF solution is conveyed to an acid removal unit, preferably in the form of an electrochemical cell, for removing HF from the acidic solution. The acid removal unit returns water to the scrubbing unit, and discharges the HF in a concentrated HF solution. This solution may be subsequently treated using calcium salts to precipitate CaF2, which may be compacted and dried for further use.
Solid particulates, such as SiO2 particulates, that are entrained within the gas stream entering the scrubbing unit are transferred to the water passing through the scrubbing unit. Some of the SiO2 particulates, for example around 30 to 60 ppm, will dissolve in the water passing through the scrubbing unit, whilst the remainder will remain as solid particulates within the water. In order to prevent these particulates from building up inside the electrochemical cell and/or the scrubbing unit, one or more filter cartridges or similar devices are provided upstream from the cell to remove these particulates from the HF solution discharged from the scrubbing unit.
Depending on the processes being conducted within the process chamber, the amount of solid particulates entrained within the gas stream may vary, typically between 70 and 200 ppm. We have found that when the amount of solid particulates is relatively high, the filter cartridges can become rapidly filled. For example, four filter cartridges each having a capacity of around 4 to 5 kg can become filled within less than a week when the gas stream contains 200 ppm of SiO2 particulates. As the cost of replacing each cartridge is currently around US$200, this can add significantly to the cost of ownership of the treatment apparatus.